The invention relates to abradably-coated walls for gas turbines, in particular for aeroengines, for example compressor casings and turbine rings, and in particular it relates to the casings of high pressure compressors and the rings of high pressure turbines.
A turbine ring is made up of a plurality of contiguous ring sectors which surround a set of rotary blades driven by the combustion gases. In order to ensure that ring sectors retain their strength at the high temperatures encountered in operation (possibly reaching 1600xc2x0 C. to 1650xc2x0 C. for a high pressure turbine), and given that the structural portions thereof are made of metal, it is known to provide them with thermal protection on their surfaces exposed to the combustion gases. Typically, the thermal protection can be constituted by a layer of refractory ceramic or metal composition conventionally formed by physical deposition, for example by thermally spraying such a deposit using a plasma.
A compressor casing also comprises one or more annular structures, which may themselves be made up of a plurality of contiguous annular sectors and which are situated facing one or more sets of blades with relative rotary movement taking place between the blades and the annular structure.
In order to ensure that the turbomachine has the best possible efficiency, it is essential to avoid or at least to minimize gas leakage between the tips of the blades and the facing surface of the compressor casing or of the turbine ring, since such leaks do not contribute to operating the turbomachine. The search for no clearance or for minimum clearance means that contact is inevitable between the tips of the blades and the material situated facing them. Because this material is conventionally very hard, such contact can destroy the tips of the blades.
In order to solve that problem, proposals have been made to improve the abradability of the material situated facing the blades, i.e. in order to improve its ability to be struck physically by the tips of the blades without giving rise to major wear thereof. One known technique for this purpose and applicable to a coating material for a turbine ring consists in imparting a high degree of porosity thereto, for example by mixing hollow beads or polymer particles with the composition used for making the material. The pores obtained by the voids created by the hollow beads or by the polymer particles disappearing at high temperature can occupy 10% to 20% or even at most about 30% of the total volume of the material. Reference can be made in particular to patent document GB 2 152 079.
Although creating pores leads to a material that is softer, and thus that presents improved abradability, that is nevertheless to the detriment of its behavior at high temperatures.
An object of the invention is to provide an annular part for a gas turbine, such as a compressor casing or a turbine ring, the part having a coating which presents a high degree of abradability, while possibly also offering effective thermal protection.
This object is achieved by an annular portion mounted facing a set of rotary gas turbine blades, said annular portion being provided with a coating of abradable material presenting cavities that open to the surface of the coating, extend over a fraction of the thickness of the coating, and are defined by walls which form a plurality of continuous paths on the inside surface of the annular portion between its axial end faces, and which, in accordance with the invention, form an angle of at least 5xc2x0 relative to the general direction of the end portions of the blades liable to come into contact with said abradable material.
Providing cavities in the coating serves to increase its abradability considerably. Because of the presence of the cavities, the external portion of the coating in which the cavities are formed can present a void content, a kind of xe2x80x9cporosityxe2x80x9d, that is greater than 40%, for example lying in the range 40% to 60%, without harming the mechanical strength of the coating.
The inclination of the walls of the cavities through an angle of not less than 5xc2x0 relative to the end portions of the blades that might come into contact with the abradable material makes it possible to limit any rubbing that might occur between the end portions and the abradable material, and thus limit wear of said end portions.
The fact that the walls of the cavities form a plurality of continuous paths between the faces of the ring serves to prevent leakage passages forming through the surface of the ring in the event of the tip of a blade penetrating into the coating, thereby ploughing a circumferential furrow passing through the walls of the cavities.
The depth of the cavities is selected to be greater than the estimated maximum penetration depth of the tips of the blades in the coating, while nevertheless remaining limited in order to avoid weakening the coating. A value lying in the range 0.3 millimeters (mm) to 1 mm is preferred.
Preferably, at least a portion of the walls have main directions forming an angle lying in the range 0xc2x0 to 20xc2x0 relative to a radial plane.
In an embodiment of the invention, the cavities are constituted by cells, preferably of polygonal section in order to conserve wall thickness that is relatively constant.
Thus, the cells may be quadrangular in section, for example they may be square and separated by walls forming a grid on the surface of the coating.
In another embodiment of the invention, the cavities are constituted by continuous circumferential grooves separated by walls that form continuous circumferential ribs extending along zigzag or undulating lines.
The invention also provides a gas turbine including a set of rotary blades surrounded by an annular portion as defined above.
The blades may have tips with chamfered rims so as to facilitate penetration into the abradable coating.